The present invention relates generally to x-ray tubes and, more particularly, to a method of fabricating and an apparatus of a rotating frame x-ray tube having a stationary cathode radially offset from a center of rotation thereof, and having a target and cathode hermetically sealed from an ambient environment.
X-ray systems typically include an x-ray tube, a detector, and a rotating assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector converts received radiation to electrical signals, and the x-ray system translates the electrical signals into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes typically include a rotatable anode structure for distributing heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into an axle that supports a disc-shaped anode target and having an iron stator structure with copper windings that surrounds the rotor. The rotor of the rotatable anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. The anode and the cathode are typically positioned within a frame that encloses a vacuum, and the frame is typically positioned within a casing that contains a coolant such as oil.
When a conventional x-ray tube is positioned in a rotatable system, such as on a CT gantry, x-rays emitting from the focal spot typically emit from a point on the anode target that is positioned radially inward, or toward the object to be imaged. This is typically accomplished by positioning the cathode within the x-ray tube at a fixed position with respect to the frame. The frame, likewise, is typically mounted within the x-ray tube casing, which is in turn mounted to a rotatable base such as that in a CT gantry. Accordingly, as the x-ray tube of a conventional design rotates about the CT gantry, the cathode emits electrons toward the target from a fixed position with respect to the x-ray tube, thus fixing the x-ray emission point (i.e., the focal spot) as well, with respect to the rotating base. In this manner, the focal spot is positioned at a constant radial position within the CT system during operation.
Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at a high rotational speed. This places stringent demands on the bearing assembly, which typically includes tool steel ball bearings and tool steel raceways positioned within the vacuum region, thereby requiring the bearing assembly to be lubricated by a solid lubricant such as silver. The rotor, as well, is typically placed in the vacuum region of the x-ray tube. Wear of the lubricant and loss thereof from the bearing contact region increases acoustic noise and slows the rotor during operation. Placement of the bearing assembly in the vacuum region prevents lubricating with wet bearing lubricants, such as grease or oil, and prevents performing maintenance on the bearing assembly to replace the solid lubricant without intrusion into the vacuum region. In addition, the operating conditions of newer generation x-ray tubes have become increasingly aggressive in terms of stresses because of g forces imposed by higher gantry speeds and higher anode rotational speeds. As a result, there is greater emphasis in finding bearing solutions for improved performance under the more stringent operating conditions.
One known solution is to position the bearings outside the vacuum region to enable use of larger, grease or oil lubricated bearings. This may be accomplished by enclosing the cathode and the anode target within a sealed volume defined by a rotatable frame. Such designs are typically referred to as “rotating frame” x-ray tubes which typically position anode target as a stationary component with respect to the frame, and the cathode is typically positioned substantially at the center of rotation of the rotating frame x-ray tube. The frame is encased in an oil bath that serves as a cooling medium to remove heat radiated from the anode target within the vacuum region to the walls of the frame. The frame is caused to rotate at a high rate of speed within the bath to prevent excessive temperatures from occurring on the target at the point of electron impingement on the target. The action of the entire frame rotating in an oil bath results in a viscous load and high demand for power in order to obtain the necessary rotation velocities.
The cathode is typically positioned at the rotational center of the frame in order to provide an emission source that remains at a central location as the frame rotates. In order to impinge electrons on the target at a position of high relative velocity to avoid overheating the focal spot, the electrons must be directed toward an outward radial position on the target. Accordingly, the electrons emitting from the cathode must be directed to the outer radial position of the target by using magnetic deflection, electrostatic deflection, and the like. As the x-ray tube is caused to rotate about the object to be imaged in the CT system, and as the frame is caused to rotate within the casing, deflection of electrons toward the target is synchronized with the rotation of the x-ray tube about the CT system, thus the focal spot is positioned at a constant radial position, directed toward the object to be imaged, within the CT system during operation.
However, the deflection mechanism within a typical rotating frame x-ray tube is difficult to implement and adds considerable cost and complexity to a CT system. Not only must a deflection mechanism be implemented, but its operation must be synchronized with rotation of the x-ray tube on the system. Furthermore, the amount of beam deflection may be limited as well. To deflect the beam an increased distance from the center-located cathode, greater electrostatic or magnetic field strength is required. Thus, a tradeoff is made between the focal spot radial position on the target that has a focal track temperature and the amount of field or electrostatic strength to accomplish the radial positioning of the focal spot. An additional tradeoff is made as well between electron deflection and distribution of the electrons on the target. Because of the severe bending that the electrons go through and the non-linear nature of the deflection mechanism, the electrons may be non-uniformly distributed on the target, thus causing the resulting focal spot to be non-uniform as well.
It would therefore be desirable to design a rotating frame x-ray tube providing dramatically improved bearing life, having a cathode at a fixed radial position with respect to a CT gantry and without having the aforementioned drawbacks of excessive field strength requirements, limited radial deflection capability of the electron beam, excess viscous drag, and non-uniform spot shapes emitting from the target.